1. Field of the Invention
The present invention relates to a method for rate conversion of coded video data and to an apparatus for the performance of the conversion; and relates in particular to a method, for rate conversion of coded video data, whereby an apparatus that records, transfers, and displays a digital moving picture requires but a simple arrangement to perform rate conversion which involves little degradation of pictures, and to an apparatus by which the method is applied.
2. Description of the Related Art
Conventional examples of highly efficient coding for moving pictures are standards methods for moving picture coding that are proposed by the ISO (International Organization For Standardization) and the JTC (Joint Technical Committee) of the IEC (International Electrotechnical Commission), i.e., MPEG1 (Moving Pictures Experts Group) and MPEG2. When the MPEG coding system is employed, a television signal is converted into coded data with a transfer speed of 1 Mbit/s to several tens of Mbit/s, in consonance with a picture quality, and is accumulated on a computer hard disk or is transmitted over a LAN (Local Area Network).
Since usable transfer speeds vary across a transfer path, such as the Internet or an ISDN, according to the busyness of lines and the contracted line speed, requests for the transfer of moving pictures are made at various speeds. Therefore, there is one technique, for example, whereby when coded data that are temporarily accumulated are present, the data are decoded to recover a picture and coding is again performed in consonance with a required transfer speed.
This technique is called a transcoding system. The outline of the transcoding system will be explained while referring to FIG. 11.
As is shown in FIG. 11, data that are coded at a coding rate R1 that is input at a coded data input terminal 51 are temporarily decoded into a picture by a decoder 52. The picture is then transmitted to a coder 53, and is coded again at a coding rate that is input via a coding rate input terminal 55. The coded data are output, via a coded data output terminal 54, as data whose coding rate has been changed to R2.
On the other hand, there are techniques whereby coding rate conversion is performed on coded data without decoding coded data into a picture. One such techniques is described in "The Study Of A Coding Rate Conversion System," by Matsumoto and Kimura, 1994 ITEJ (The Institute of Television Engineers of Japan) Annual Convention, pp. 183 to 184. This system will be briefly explained while referring to FIG. 12.
As is shown in FIG. 12, a variable length decoder 62 extracts DCT (Discrete Cosine Transform) coding information from data that have been coded at a coding rate R1 that is input via a coded data input terminal 61. One part of the DCT coding information is selected and the remainder is deleted by a DCT coefficient selector 63 in accordance with a coding rate that is input via a coding rate input terminal 66. The selected DCT coding information is coded by a variable length coder 64, and the resultant data is output, via a coded data output terminal 65, as coded data whose coding rate has been changed to R2. This system is called DCT coefficient partitioning.
The DCT coefficient partitioning will be described in detail while referring to FIG. 13. Although a coding process is generally performed while employing an 8.times.8 pel grid as one block, for simplicity a 4.times.4 pel block is shown in FIG. 13. In FIG. 13, assuming that reference number 71 denotes a coefficient matrix that is obtained by performing a DCT process on a 4.times.4 pel block (not shown), the elements of the coefficient matrix 71 are divided by corresponding elements in a quantization matrix 72. The remainder of this division is omitted, and a quantized coefficient matrix 74 (i.e., the above described coded information) is acquired from the integer portion. The elements of the coded information 74 are rearranged in the order of the elements in a zig-zag scan matrix 75, and thus a quantized coefficient matrix 76 is acquired. Then, entropy coding is performed on the quantized coefficient matrix 76.
According to the DCT coefficient partitioning, the variable length decoder 62 decodes the data acquired by performing entropy coding while the DCT coefficient selector 63 selects, for example, only a part 74a of the coded information 74 and deletes the remainder of the coded information. In this manner, the DCT coded information 74a that was selected is coded by the variable length coder 64, and the resultant data are output, via the coded data output terminal 65, as coded data whose coding rate has been changed to R2.
According to the description in the above document, however, assuming that the original coding rate R1 is 6 Mbit/s and the converted rate R2 is 4 Mbit/s, the transcoding in MPEG2 is degraded by about 1.5 dB as compared with normal coding at 4 Mbit/s. The term normal coding means the coding that is ordinarily performed without rate conversion, i.e., the coding system in which a picture is directly coded by a coder. The transcoding requires the decoder 52 and the coder 53, so that the size of the arrangement for an apparatus is increased and the manufacturing cost of the apparatus is also raised.
Since the DCT coefficient partitioning can be obtained by using the variable length decoder 62, the DCT coefficient selector 63, and the variable length coder 64, the arrangement for an apparatus is very simple. According to the description in the above document, however, the DCT coefficient partitioning is degraded by about 3 dB compared with the results that are obtained with normal coding. Therefore, with the DCT coefficient partitioning, conversion causes a picture to be deteriorated very much. The cause of the deterioration in a picture is regarded as being the deletion of information other than the part 74a of the DCT coded information 74 in FIG. 13, for example.